It is of course well known to form elongated hollow bodies of considerable size of concrete for use as caissons, tunnel sections, etc . . . More recently, the prior patents record suggestions for forming closed-ended tanks designed to transport under tow and/or to store under water, petroleum or other lighter-than-water fluids, examples for such patents being Georgii U.S. Pat. No. 3,249,664 dated May 3, 1966, and Shurtleff U.S. Pat. No. 3,435,793 dated Apr. 1, 1969. However, such patented suggestions in the light of present-day technology appear to be visionary only and to constitute but superficial and impractical approaches to the problem, rather than teaching methods and/or apparatus which in fact advance the particular art.